I suspect it probably depends on the exact makeup of the raw meal
Red meat is likely to take a bit longer than white meat, for example, and bones a bit longer than cartillage. Veggies and other plant matter take longer still (and actually largely get passed undigested). 8 hours is the time generally given as a "working average" though, so you could probably plan around that.
For kibble, the digestion time is generally thought to be in the 12-14 hour range. In both cases though, food has probably left the stomach and is in the large intestine for further digestion (or at least, one would assume so).
What you can probably figure from all that is that whilst you could probably get away with feeding a kibble meal around 6-8 hours after raw, a raw meal shouldn't really follow a kibble one for 10-12 hours or so. So if you're looking to reduce time between meals, the order in which you feed them does matter. If you want a shorter space of time, feed the raw first (and if you want longer, feed the kibble meal first).